Rotary well drilling for oil and gas is primarily accomplished through one of two types of bits. In a rotary cutter bit, the bit body has typically three rotatable cones or cutters. The cones rotate on bearing pins and have teeth or tungsten carbide inserts for disintegrating the earth formation. In the fixed cutter or drag bit type, the bit body has a face which contains cutting elements mounted on fixed blades. The cutting elements are typically polycrystalline diamond. The bit body has drilling fluid passages with nozzles for discharging drilling fluid through junk slots that are located between the blades.
Drag bits are extensively used in directionally drilling, particularly in the technique referred to as steerable drilling. In this method, the drill bit is steered in desired directions for cutting borehole segments as it progresses. A mud motor or turbine is employed with the bit assembly for rotating the drag bit while the drill string remains stationary.
In the prior art, drag bits used for steerable drilling have employed relatively short gage lengths, often even shorter than gage lengths for conventional bits not used for steerable applications. The gage length is a portion of the bit body that extends upward from the face and has an effective diameter approximately equal to the diameter of the borehole being cut. On the other hand, the parent application to this application explains that short gage bits also produce an increased amount of borehole irregularities, such as side wall ledging, spiraling of the borehole and rifling of the borehole sidewall. Excessive side cutting of a bit may lead to ledging of a severity such that downhole tools may actually become stuck while travelling through the borehole.
The parent application teaches to provide a longer gage length with tandem gage pads along the gage surface of the bit body. In the parent application, the gage pads are shown to be integrally formed with the bit body. Drag bit bodies of this nature are typically formed of a carbide matrix, such as tungsten carbide. While tungsten carbide works well for drag bit bodies generally, problems may occur if the bit body is lengthened to accommodate tandem gage pads. Tungsten carbide does not withstand excessive bending loads well, thus cracking may tend to occur in the body under extreme loading conditions.